Active termination and switchable passive termination circuits

ABSTRACT

According to one exemplary embodiment, an active termination circuit includes at least one active termination branch, where the at least one active termination branch includes at least one transistor for providing an active termination output. The at least one active termination branch further includes an amplifier driving the at least one transistor, where the amplifier has a non-inverting input coupled to the active termination output via a feedback network. The amplifier controls a current flowing through the at least one transistor so as to provide the active termination output. The active termination output can be provided at a drain of the at least one transistor, where a source of the at least one transistor is coupled to ground through a degeneration transistor and a tail current sink.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of electrical circuits.More particularly, the invention is in the field of transmission linetermination circuits.

2. Background Art

Transmitter modules, which can include line drivers, can be utilized inset-top boxes, cable modems, routers, computer interfaces, and otherelectronic devices to drive transmission lines, such as Ethernettransmission lines. For example, a transmitter module including a linedriver can be utilized in an Ethernet interface in a computer. Toprevent return loss and signal distortion, for example, a transmittermodule or a line driver in the transmitter module is typically requiredto provide an output impedance that matches the characteristic impedanceof the transmission line. For example, a line driver in an Ethernetapplication can be required to provide a 50.0 ohm output impedance todrive a 50.0 ohm load via a 50.0 ohm Ethernet cable coupled to anEthernet port. Conventionally, a transmitter module can be passivelyterminated by utilizing one or more termination resistors to provide arequired output impedance. However, the conventional approach toproviding a termination resistance for transmitter modules or linedrivers in the transmitter modules can have undesirably consequences.

Conventionally, for example, a line driver, such as a current-based linedriver, with differential outputs in a transmitter module can bepassively terminated by utilizing two termination resistors coupledacross the positive and negative outputs of the line driver. In anEthernet application, for example, each of the termination resistors canhave a resistance of approximately 50.0 ohms to provide approximately100.0 ohms across the differential positive and negative outputs of theline driver. However, when the line driver is transmitting data over atransmission line, a large amount of the current generated by the linedriver can flow through the termination resistors and remain on-chip,thereby undesirably increasing on-chip current consumption. As a result,heat generated by the current flowing through the termination resistorscan increase packaging cost required to handle the additional heat.Also, the line driver can require a larger layout area to handle theadditional current required, which can undesirably increasemanufacturing cost.

SUMMARY OF THE INVENTION

Active termination and switchable passive termination circuitssubstantially as shown in and/or described in connection with at leastone of the figures, and as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a circuit diagram of an exemplary conventional linedriver.

FIG. 2 illustrates a circuit diagram of an exemplary active terminationcircuit in accordance with one embodiment of the present invention.

FIG. 3 illustrates a circuit diagram of an exemplary line driver inaccordance with one embodiment of the present invention.

FIG. 4 illustrates a circuit diagram of an exemplary switchable passivetermination circuit in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to active termination and switchablepassive termination circuits. The following description containsspecific information pertaining to the implementation of the presentinvention. One skilled in the art will recognize that the presentinvention may be implemented in a manner different from thatspecifically discussed in the present application. Moreover, some of thespecific details of the invention are not discussed in order not toobscure the invention.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. To maintain brevity, other embodiments of the presentinvention are not specifically described in the present application andare not specifically illustrated by the present drawings.

FIG. 1 shows a circuit diagram of an exemplary conventional line drivercoupled to a pair of termination resistors. In FIG. 1, conventional linedriver 102 is coupled to termination resistors 104 and 106 viatransformer 108. Termination resistors 104 and 106 can represent, forexample, an RJ45 Ethernet port in a receiver module. Conventional linedriver 102, which can be a conventional current-based line driver,includes transistors 110 and 112, termination resistors 114 and 116,tail current sink 118, which includes transistors 120 and 122, and biascircuit 124, which includes transistors 126, 128, 130, and 132.Conventional line driver 102 can be utilized in set-top boxes, cablemodems, routers, computers, and other electronic devices to drivetransmission lines coupled to the devices. For example, conventionalline drive 102 can be utilized in any IC chip with an Ethernetinterface, where the Ethernet interface is coupled to a Ethernettransmission line, such as a category (CAT) 5 Ethernet cable.Conventional line driver 102 can be fabricated on a semiconductor die(also referred to as a “chip” or simply as a “die” in the presentapplication).

Conventional line driver 102 can be configured to receive data negativeinput (DN IN) 134 and data positive input (DP IN) 136 and can providevoltage positive output (VP OUT) 138 and voltage negative output (VNOUT) 140, where VP OUT 138 and VN OUT 140 are differential outputs. DNIN 134 and DP IN 136 are digital inputs, where DN IN 134 is thecomplement of DP IN 136. Also, data can be inputted into conventionalline driver 102 as a digital word comprising multiple bits, where eachbit and its complement can be inputted into respective transistors, suchas transistors 112 and 110, which are coupled to a tail current sink,such as tail current sink 118. Thus, for a digital word comprising 16bits, the configuration of transistors 112 and 110 and tail current sink118 can be repeated 16 times, where the drain of each transistor (e.g.transistor 112) receiving a bit of the digital word can be coupled tonode 144, the drain of each transistor (e.g. transistor 110) receiving acomplementary bit of the digital word can be coupled to node 142, andeach tail current sink, such as tail current sink 118, can be coupled tobias circuit 124.

As shown in FIG. 1, bias circuit 124 receives bias current (IBIAS) 146and IBIAS 148 on respective lines 150 and 152 and provides bias voltagesfor respective transistors 120 and 122 on lines 154 and 156. Also shownin FIG. 1, transistors 126 and 128 are coupled in series between line150 and ground 158 and transistors 130 and 132 are coupled in seriesbetween line 152 and ground 158, where transistors 126, 128, 130, and132 can each be an N-channel field effect transistor (NFET). Furthershown in FIG. 1, transistors 120 and 122, which can be NFETs, arecoupled in series between ground 158 and node 160, at which the sourcesof transistors 110 and 112 are coupled. Transistors 120 and 122 incurrent tail sink 118 can draw tail current (ITAIL) 162 throughtransistors 110 and 112. ITAIL 162 can be determined by the biasvoltages provided by bias circuit 124 on lines 154 and 156. For a 10Base T (10BT) Ethernet data transmission standard, ITAIL 162 can be, forexample, approximately 100.0 milliamperes (mA). For a 100TX Ethernetdata transmission standard, ITAIL 162 can be, for example, approximately40.0 mA.

Also shown in FIG. 1, the drains of transistors 110 and 112 are coupledto respective nodes 142 and 144, which provide VP OUT 138 and VN OUT140, termination resistors 114 and 116 are coupled in series betweennodes 142 and 144, and DN IN 134 and DP IN 136 are coupled to therespective gates of transistors 110 and 112, which can be NFETs.Termination resistors 114 and 116 can provide on-chip passivetermination for conventional line driver 102 and can each have aresistance equal to, for example, approximately 50.0 ohms. Further shownin FIG. 1, primary winding 164 of transformer 108 is coupled acrossnodes 142 and 144, supply voltage 166 (i.e. VDD) is coupled to thecenter tap of primary winding 164 at node 168, and termination resistors104 and 106 are coupled in series across secondary winding 170 oftransformer 108 via transmission line 172, which can be an Ethernettransmission line. Termination resistors, which can each be equal toapproximately 50.0 ohms, can represent, for example, an RJ45 Ethernetport in a receiver module. Primary winding 164 includes inductors 165and 167, which provide VDD to respective nodes 142 and 144.

Conventional line driver 102 can provide output impedance ZOUT 174 atnode 142 and ZOUT 176 at node 144 as required to match thecharacteristic impedance of the transmission line that the line driveris driving, such as transmission line 172. For example, ZOUT 174 andZOUT 176 can each be equal to approximately 50.0 ohms. The outputimpedance of line driver 102 can be provided through on-chip passivetermination provided by termination resistors 114 and 116, which arecoupled between output nodes 142 and 144. Termination resistors 114 and116 can provide an output impedance between differential outputs VP OUT138 and VN OUT 140 at respective nodes 142 and 144 that can be equal to,for example, approximately 100.0 ohms.

However, when conventional line driver 102 is transmitting data overtransmission line 172, a large amount of output current is shuntedthrough (i.e. consumed by) termination resistors 114 and 116. Forexample, approximately 50.0 percent of the output current generated byconventional line driver 102 can be consumed by termination resistors114 and 116. Thus, termination resistors 114 and 116 can significantlyincrease on-chip power consumption, which can require a more expensivepackage to handle the additional heat generated by the terminationresistors. Also, to handle the additional current shunted throughtermination resistors 114 and 116, transistors 110 and 112 and tailcurrent sink 118 each need to have a larger size, thereby increasing thelayout area of conventional line driver 102 and, consequently,increasing manufacturing cost.

FIG. 2 shows a circuit diagram of an exemplary differential activetermination circuit in accordance with one embodiment of the presentinvention. Active termination circuit 202, which is a differentialactive termination circuit, includes active termination branches (alsoreferred to simply as “branches”) 202 and 204 and tail current sink 206.Branch 202 includes amplifier 208, transistor 210, feedback resistors212 and 214, and degeneration resistor 216 and branch 204 includesamplifier 218, transistor 220, feedback resistors 222 and 224, anddegeneration resistor 226. In one embodiment, each component in branch202 is substantially identical to the corresponding component in branch204. Thus, amplifier 208 in branch 202 is substantially identical toamplifier 218 in branch 204, transistor 210 in branch 202 issubstantially identical to transistor 220 in branch 204, and so forth.Branch 202 is coupled to branch 204 at node 228 and tail current sink206 is coupled between node 228 and ground 230.

As shown in FIG. 2, a first terminal of tail current sink 206 is coupledto ground 230 and a second terminal of tail current sink 206 is coupledto first terminals of degeneration resistors 216 and 226 at node 228.Degeneration resistors 216 and 226 can each have resistance equal to,for example, approximately 4.5 ohms in one embodiment. Tail current sink206 sinks tail current (ITAIL) 231, which is substantially equal to thecurrent flowing through transistors 210 and 220. For a 10BT Ethernetdata transmission standard, ITAIL 231 can be, for example, approximately55.0 mA. For a 100TX Ethernet data transmission standard, ITAIL 231 canbe, for example, approximately 25.0 mA.

Also shown in FIG. 2, a second terminal of degeneration resistor 216 iscoupled to the source of transistor 210 and the negative (inverting)input of amplifier 208 at node 232, and the second terminal ofdegeneration resistor 226 is coupled to the source of transistor 220 andthe negative (inverting) input of amplifier 218 at node 234. Amplifiers208 and 218 can each be an operational amplifier, such as a high gainoperational amplifier. Transistors 210 and 220 can each be an NFET, forexample. In one embodiment, more than one transistor can be utilized inplace of each of transistors 210 and 220. Further shown in FIG. 2, afirst terminal of feedback resistor 212 is coupled to ground 230 and asecond terminal of feedback resistor 212 is coupled to a first terminalof feedback resistor 214 and the positive (non-inverting) input ofamplifier 208.

Also shown in FIG. 2, a first terminal of feedback resistor 222 iscoupled to ground 230 and a second terminal of feedback resistor 222 iscoupled to a first terminal of feedback resistor 224 and the positive(non-inverting) input of amplifier 218. Feedback resistors 212 and 222can each be an adjustable resistor and can have a resistance equal to,for example, approximately 1.0 kilo-ohm in one embodiment. Feedbackresistors 214 and 224 can each have a resistance equal to, for example,at least 10.0 kilo-ohms. Further shown in FIG. 2, the output ofamplifier 208 is coupled to the gate of transistor 210 and the drain oftransistor 210 is coupled to a second terminal of feedback resistor 214and output terminal 236 at node 238. Also shown in FIG. 2, the output ofamplifier 218 is coupled to the gate of transistor 220 and the drain oftransistor 220 is coupled to a second terminal of feedback resistor 224and output terminal 240 at node 242.

The operation of branch 202 of active termination circuit 200 will nowbe discussed. Since branch 204 of active termination circuit 200operates in a substantially similar manner as branch 202, the followingdiscussion also applies to branch 204. In the following discussion, itis assumed that:

g _(m) ·R _(TERM)>>1.0   equation (1)

where “g_(m)” is the transconductance of transistor 210 and “R_(TERM)”is the resistance of degeneration resistor 216. As a result of thevoltage divider comprising feedback resistors 212 and 214,V₍₊₎=V⁽⁻⁾=V₂₃₈·R₁/(R₁+R₂), where “V₍₊₎” is the voltage at the positiveterminal of amplifier 208, “V⁽⁻⁾” is the voltage at the negativeterminal of amplifier 208, “V₂₃₈” is the voltage at node 238 (i.e. atoutput terminal 236), “R₁” is the resistance of feedback resistor 212,and “R₂” is the resistance of feedback resistor 214. Also, the feedbacknetwork provided by feedback resistors 212 and 214 ensures thatV₍₊₎=V⁽⁻⁾.

Since the negative input of amplifier 208 has a high input impedance, it(i.e. the negative input of amplifier 208) draws a negligible amount ofcurrent. Therefore, I_(RTERM)=V₂₃₈/R_(TERM), where “I_(RTERM)” is thecurrent flowing through degeneration resistor 216. As a result, outputimpedance (ZOUT) at node 238, which is indicated by arrow 244, can berepresented by the equation:

ZOUT (at node 238)=((R ₁ +R ₂)/R ₁)·R _(TERM)   equation (2)

By utilizing amplifier 208 to drive transistor 210, amplifier 208effectively increases the transconductance (g_(m)) of transistor 210,which helps to insure that the condition in equation (1) is satisfied.By insuring that the condition in equation (1) is satisfied, ZOUT atnode 238 (indicated by arrow 244) is less dependent on the value ofg_(m) and more dependent on resistance R_(TERM), which is desirablesince the properties of transistor 210 are more dependent on variationsin process, voltage, and temperature than the resistance (i.e. R_(TERM))of degeneration resistor 216. Thus, ZOUT at node 238 has a reducedvariation dependency on process, temperature, and voltage, which causesan output signal at output terminal 236 to have increased linearity.Since branch 204 is substantially identical to branch 202, ZOUT at node242 (indicated by arrow 246) is also represented by equation (2). Sinceactive termination circuit 200 operates in a differential mode (i.e. ithas differential outputs at output terminals 236 and 240), ZOUT acrossnodes 238 and 242 can be represented by the equation:

ZOUT (across nodes 238 and 242)=2.0·((R ₁ +R ₂)/R ₁)·R _(TERM)  equation (3)

In an Ethernet application, for example, ZOUT at each of nodes 238 and242 can be equal to approximately 50.0 ohms and ZOUT across nodes 238and 242 (i.e. across differential outputs at output terminals 236 and240) can be equal to approximately 100.0 ohms. The values of ZOUT atnodes 238 and 242 can be adjusted (i.e. trimmed) by appropriatelyadjusting respective feedback resistors 212 and 222, which enablesactive termination circuit 200 to be programmed. In one embodiment, thevalues of feedback resistors 212 and 222 can be predetermined prior tofabrication of active termination circuit 200 on a die.

By providing amplifiers 208 and 218 to drive the respective gates oftransistors 210 and 220, providing feedback resistors 212, 214, 222, and224, and providing degeneration resistors 216 and 226 coupled in serieswith transistors 210 and 220, an embodiment of the invention's activetermination circuit 200 can provide an active termination output atnodes 238 and 242 in a differential current-mode line driver. Activetermination circuit 200 can provide, for example, an output impedancebetween nodes 238 and 242 equal to approximately 100.0 ohms in adifferential operating mode without requiring a shunt resistance betweennodes 238 and 242, thereby significantly reducing current consumption.In contrast, in conventional line driver 102, termination resistors 114and 116, which are coupled between nodes 142 and 144, effectivelyprovide a shunt resistance that consumes current that would otherwise beutilized to drive transmission line 172, which is coupled todifferential outputs VP OUT 138 and VN OUT 140 via transformer 108.

Thus, an embodiment of the invention in FIG. 2 provides a differentialactive termination circuit that provides active termination outputswhile consuming significantly less power than the passive terminationprovided by termination resistors 114 and 116 in conventional linedriver 102 in FIG. 1. By reducing on-chip current consumption,transistors 210 and 220 can be smaller in size, which advantageouslyreduces the die area consumed by active termination circuit 200, therebyadvantageously reducing manufacturing cost. Also, by eliminating on-chiptermination resistors, an embodiment of the invention's activetermination circuit eliminates the heat generated by the on-chiptermination resistors, thereby advantageously eliminating the need formore expensive packaging that could be required to handle the additionalheat generated by the termination resistors. Also, since feedbackresistors 212 and 222 are adjustable, the output impedance across nodes238 and 242 can be advantageously trimmed by appropriately adjustingfeedback resistors 212 and 222.

FIG. 3 shows a circuit diagram of an exemplary line driver in accordancewith one embodiment of the present invention. Line driver 300 includesactive termination circuit 302 and current digital-to-analog converter(DAC) 304. Active termination circuit 302, which is a differentialactive termination circuit, corresponds to active termination circuit200 in FIG. 2. In particular, tail current sink 306, amplifiers 308 and318, transistors 310 and 320, feedback resistors 312, 314, 322, and 324,degeneration resistors 316 and 326, nodes 328, 338, and 342, and ground330 in active termination circuit 302 correspond, respectively, to tailcurrent sink 206, amplifiers 208 and 218, transistors 210 and 220,feedback resistors 212, 214, 222, and 224, degeneration resistors 216and 226, nodes 228, 238, and 242, and ground 230 in active terminationcircuit 200 in FIG. 2. Current DAC 304 includes transistors 350 and 352and tail current sink 354. Line driver 300 can be a current-based linedriver, such as a current-based Ethernet line driver. Line driver 300can be utilized, for example, in set-top boxes, cable modems, routers,computers, and other electronic devices to drive data on transmissionlines, such an Ethernet cables. For example, line driver 300 can beutilized in any IC chip with an Ethernet interface, which can be coupledto an Ethernet transmission line, such as a CAT 5 Ethernet cable orother type of Ethernet cable.

Line driver 300 can be configured to receive data negative input (DN IN)356 and data positive input (DP IN) 358 and to provide voltage positiveoutput (VP OUT) 360 and voltage negative output (VN OUT) 362, where VPOUT 360 and VN OUT 362 are differential outputs and DN IN 356 is thecomplement of DP IN 358. Thus, if DP IN 358 provides a digital value of“1,” DN IN 356 will provide a digital value of “0,” and vice versa.Also, data can be inputted into line driver 300 as a multi-bit digitalword and its complement, where each bit requires a separate current DAC,such as current DAC 304. Thus, for example, in an embodiment in which adigital word comprising 16 bits is inputted into line driver 300, 16current DACs, such as current DAC 304, would be coupled to activetermination circuit 302, where each current DAC would correspond to onebit of the digital word. In the above example, each current DAC woulddrive active termination circuit 302 concurrently and the currentoutputs provided by the current DACs would be summed together.

As shown in FIG. 3, a first terminal of tail current sink 354 is coupledto ground 330 and a second terminal of tail current sink 354 is coupledto the sources of transistors 350 and 352 at node 364. Transistors 350and 352 can each be an NFET, for example. Also shown in FIG. 3, DN IN356 is coupled to the gate of transistor 350 and the drain of transistor350 is coupled to the positive (non-inverting) input of amplifier 308.Further shown in FIG. 3, DP IN 358 is coupled to the gate of transistor352 and the drain of transistor 352 is coupled to the positive(non-inverting) input of amplifier 318. Thus, the current outputsprovided by current DAC 304 are coupled to the respective positiveinputs of amplifiers 308 and 318 of active termination circuit 302.Current (I_(DAC)) 366, which is the current sunk by current DAC 304,determines the amplitude of the current outputs provided to activetermination circuit 302 by current DAC 304.

Line driver 300 can provide an active termination output at nodes 338and 342, where the output impedance between nodes 338 and 342 can bedetermined by equation (3). In an Ethernet application, for example,line driver 300 can provide an active termination output ofapproximately 100.0 ohms across nodes 338 and 342, i.e., acrossdifferential outputs VP OUT 360 and VN OUT 362. By including activetermination circuit 302, line driver 300 provides similar advantages aspreviously discussed with respect to active termination circuit 200 inFIG. 2, such as reduced on-chip current consumption and reduced areaconsumed on the die. As apparent from the above description, theembodiments of the invention in FIGS. 2 and 3 can be generally utilizedin a transmitter while the transmitter is in a transmit mode.

FIG. 4 shows a circuit diagram of an exemplary switchable passivetermination circuit in accordance with one embodiment of the presentinvention. Switchable passive termination circuit 400 (also referred tosimply as “passive termination circuit 400”) includes transistors 402,404, 406, and 408, termination resistors 410 and 412, and resistor 414.Passive termination circuit 400 can be utilized, for example, to provideswitchable passive termination for a transmitter module, which caninclude a line driver, such as line driver 300 in FIG. 3, coupled to atransmission line, such as an Ethernet cable. When the transmittermodule is transmitting data over the transmission line, the transmittermodule can utilize an embodiment of the invention's active terminationcircuit (e.g. active termination circuit 200 in FIG. 2) to providedifferential active termination and the passive termination provided bypassive termination circuit 400 can be switched off. When thetransmitter module is not transmitting data over the transmission line,passive termination can be switched on across the differential outputsof the transmitter module by passive termination circuit 400.

As shown in FIG. 4, the source of transistor 408 is coupled to ground416, control signal 418 is coupled to the gate of transistor 408, andthe drain of transistor 408 is coupled to the source of transistor 406.Transistor 408, which can be, for example, an NFET, can be configured asa switch that can be turned on or off in response to control signal 418,which can be a digital signal. For example, control signal 418 canprovide a digital value of “1” to turn on transistor 408, where thedigital value of “1” can be represented by VDD, and can provide adigital value of “0” to turn off transistor 408, where the digital valueof “0” can be represented by 0.0 volts (e.g. ground potential) in oneembodiment.

Also shown in FIG. 4, bias voltage (VBIAS) 420 is coupled to the gate oftransistor 406 and the drain of transistor 406 is coupled to the gatesof transistors 402 and 404 and a first terminal of resistor 414 at node422. Transistor 406 can function as a voltage protect transistor and canbe, for example, an NFET. VBIAS 420 can provide a bias voltage equal to,for example, VDD to the gate of transistor 406. Resistor 414 can have aresistance equal to, for example, approximately 25.0 kilo-ohms in oneembodiment. Each of transistors 402 and 404 can be a P-channel FET(PFET), for example. Each of transistors 402 and 404 can be, forexample, a very large transistor having a low on-resistance such thatthe transistor has a very small source-to-drain voltage when turned on.

Further shown in FIG. 4, a second terminal of resistor 414 is coupled toa source and a bulk terminal of transistor 402 and a source and a bulkterminal of transistor 404 at node 424. Also shown in FIG. 4, a drain oftransistor 402 is coupled to a first terminal of termination resistor410 and a second terminal of termination resistor 410 is coupled toterminal 426 of passive termination circuit 400. Termination resistor410 can have a resistance equal to, for example, approximately 50.0 ohmsin one embodiment. Further shown in FIG. 4, a drain of transistor 404 iscoupled to a first terminal of termination resistor 412 and a secondterminal of termination resistor 412 is coupled to terminal 428 ofswitchable passive termination circuit 400. Termination resistor 412 canhave a resistance equal to, for example, approximately 50.0 ohms in oneembodiment.

Passive termination circuit 400 can be coupled across differentialoutputs of a transmitter module, such as an open-drain transmittermodule, where the outputs of the transmitter module can be coupledacross a transmission line, such as an Ethernet cable. The transmittermodule can be situated in an electronic device that also includes areceiver module, where the electronic device can be, for example, aset-top box, a cable modem, a router, a computer interface, or othertype of communications device. For example, the electronic device caninclude an Ethernet interface coupled to an Ethernet transmission line,which can be driven by the transmitter module. In one embodiment, thetransmitter module can include a line driver, such as line driver 300 inFIG. 3. Passive termination circuit 400 can be configured to operate ina transmit mode when the transmitter module is transmitting data overthe transmission line, such as an Ethernet transmission line, and tooperate in a receive mode when the transmitter module is nottransmitting data over the transmission line.

A more detailed operation of passive termination circuit 400 in thereceive and transmit modes will now be discussed. To illustrate theoperation of passive termination circuit 400, terminals 426 and 428 caneach have a voltage that swings above and below VDD, which can beapproximately 2.5 volts. Thus, in the following discussion, terminal 426can swing between 1.25 volts and 3.75 volts and terminal 428 can swingbetween 3.75 volts and 1.25 volts, for example. In the receive mode,control signal 418 can apply a sufficiently high voltage level, such aVDD, to the gate of transistor 408 to turn it (i.e. transistor 408) onand VBIAS 420, which can be, for example approximately equal to VDD, canturn on transistor 406. When transistors 406 and 408 are turned on, thegates of transistors 402 and 404 are pulled to ground, which can be, forexample, at approximately 0.0 volts. Since resistor 414 has a highresistance, it only draws a minimal amount of current. As a result, node424 can have a voltage equal to approximately 2.5 volts (i.e. VDD in thepresent example). When the gates of transistors 402 and 404 are pulledto ground, transistors 402 and 404 are turned on (i.e. conducting),thereby providing a very low impedance path between terminationresistors 410 and 412. As a result, termination resistors 410 and 412are electrically coupled between terminals 426 and 428.

In the receive mode, all transistors in passive termination circuit 400are required to operate in a “safe” region, where “safe” is defined soas to eliminate transistor breakdown. In the present example, in whichVDD is equal to approximately 2.5 volts, “safe” is defined to requirethe gate-to-drain voltage, gate-to-source voltage, source-to-drainvoltage, and gate-to-bulk voltage of each transistor in passivetermination circuit 400 to be less than 2.5 volts.

In the transmit mode, control signal 418 can apply a sufficiently lowvoltage level, such as 0.0 volts, to the gate of transistor 408 to turnit off and VBIAS 420 can apply a sufficient high voltage level to thegate of transistor 406 to turn it on. By turning off transistor 408,current is prevented from flowing to ground 416. As a result, resistor414 can pull up node 422 to the substantially the same voltage as node424. Since transistors 402 and 404 are PFETs, the gate-to-source voltageof transistor 402 and transistor 404 is equal to approximately 0.0volts. As a result, one of transistors 402 and 404 will be turned off,thereby providing a high impedance path between termination resistors410 and 412 and, consequently, between terminals 426 and 428 of passivetermination circuit 400. For example, the transistor coupled to theterminal of passive termination circuit 400 having a low voltage swingwill be turned off and the transistor coupled to the terminal of passivetermination circuit 400 having a high voltage swing will be conducting.As a result, termination resistor 410 will be electrically decoupledfrom termination resistor 412 and a high impedance will be providedbetween terminals 426 and 428 of passive termination circuit 400.

In the transmit mode, all transistors in passive termination circuit 400are also required to operate in the “safe” region, as previouslydefined. In the present example node 424 can be as high as 3.75 volts,since it (i.e. node 424) follows the highest voltage provided atterminals 426 and 428 of passive termination circuit 400. Since node 422follows node 424, it (i.e. node 422) can be as high as 3.75 volts in thepresent example. Thus, if transistor 406 was not utilized and the drainof transistor 408 was directly coupled to node 422, the drain oftransistor 408 would see 3.75 volts, which would have a degrading effecton transistor 408. However, transistor 406 protects transistor 408 bypreventing the drain of transistor 408 from rising above 2.5 volts.

Thus, in the embodiment in FIG. 4, passive termination circuit 400 canbe configured to operate in a transmit mode and a receive mode and canbe coupled across differential outputs of a transmitter module, whichcan be coupled across a transmission line, such as an Ethernettransmission line. In the transmit mode, passive termination circuit 400can provide a high impedance between terminals 426 and 428 byelectrically decoupling termination resistors 410 and 412, and in thereceive mode, passive termination circuit 400 can provide passivetermination between terminals 426 and 428 by electrically couplingtermination resistors 410 and 412 between terminals 426 and 428. Thus,passive termination circuit 400 advantageously reduces on-chip currentconsumption by preventing current from flowing through terminationresistors 410 and 412 in the transmit mode.

Thus, as discussed above, in an embodiment in FIG. 2, the inventionprovides an active termination circuit, such as a differential activetermination circuit, for providing active termination across the outputsof a line driver, such as an open-drain Ethernet line driver, where theactive termination circuit advantageously reduces on-chip currentconsumption. As a result, an embodiment of the invention's activetermination circuit can utilize smaller size transistors, which canadvantageously reduce the die area consumed by the active terminationcircuit, thereby reducing manufacturing cost. In an embodiment in FIG.3, the invention provides a line driver, such as an open-drain Ethernetline driver, that advantageously utilizes the invention's activetermination circuit. As apparent from the above description, theembodiments of the invention in FIGS. 2 and 3 can be generally utilizedin a transmitter while the transmitter is in a transmit mode.

In an embodiment in FIG. 4, the invention provides a switchable passivetermination circuit that is configured to operate in a receive mode anda transmit mode. In the receive mode, an embodiment of the invention'sswitchable passive termination circuit can provide passive terminationacross differential outputs of a transmitter module when the transmittermodule is not transmitting data across a transmission line, such as anEthernet transmission line. In the transmit mode, the embodiment of theinvention's switchable passive termination circuit can provide a highimpedance across the differential outputs of the transmitter module whenthe transmitter module is transmitting data across the transmissionline. Thus, the invention's switchable passive termination circuitadvantageously reduces on-chip current by preventing current fromflowing through the passive termination in the transmit mode.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skill in the art would appreciate thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. Thus, the described embodiments are tobe considered in all respects as illustrative and not restrictive. Itshould also be understood that the invention is not limited to theparticular embodiments described herein but is capable of manyrearrangements, modifications, and substitutions without departing fromthe scope of the invention.

1. An active termination circuit comprising at least one activetermination branch and an active termination output, said at least oneactive termination branch comprising: at least one transistor forproviding said active termination output, a source of said at least onetransistor being coupled to a degeneration resistor; an amplifierdriving said at least one transistor, said amplifier having anon-inverting input coupled to said active termination output via afeedback network, said amplifier controlling a current flowing throughsaid at least one transistor to provide said active termination output.2. The active termination circuit of claim 1, wherein said activetermination output is provided at a drain of said at least onetransistor.
 3. The active termination circuit of claim 1, wherein saidsource of said at least one transistor is coupled to ground through saiddegeneration resistor and a tail current sink.
 4. The active terminationcircuit of claim 1, wherein an inverting input of said amplifier iscoupled to said source of said at least one transistor.
 5. The activetermination circuit of claim 1, wherein said feedback network comprisesa first feedback resistor coupled between said non-inverting input ofsaid amplifier and ground.
 6. The active termination circuit of claim 5,wherein said feedback network further comprises a second feedbackresistor coupled between said non-inverting input of said amplifier andsaid active termination output.
 7. The active termination circuit ofclaim 5, wherein said first feedback resistor is adjustable for trimmingan output impedance at said active termination output. 8-13. (canceled)14. A line driver comprising: an active termination circuit includingtwo active termination branches, each of said active terminationbranches comprising: at least one transistor for providing an activetermination output; an amplifier driving said at least one transistor,said amplifier having a non-inverting input coupled to said activetermination output via a feedback network; said amplifier controlling acurrent flowing through said at least one transistor so as to providesaid active termination output.
 15. The line driver of clam 14 furthercomprising at least one current DAC coupled to said active terminationcircuit, wherein said at least one current DAC has a data negative inputand a data positive input.
 16. The line driver of claim 14, wherein saidactive termination output is provided by a drain of said at least onetransistor, and wherein a source of said at least one transistor iscoupled to ground through a degeneration transistor.
 17. The line driverof clam 14, wherein said active termination circuit further comprises atail current sink coupled between said two active termination branchesand ground.
 18. The line driver of claim 14, wherein said feedbacknetwork comprises a first feedback resistor coupled between saidnon-inverting input of said amplifier and said active termination outputand a second feedback resistor coupled between said non-inverting inputof said amplifier and ground.
 19. The line driver of claim 18, whereinsaid second feedback resistor is adjustable for trimming an outputimpedance at said active termination output.
 20. The line driver ofclaim 16, wherein an inverting input of said amplifier is coupled tosaid source of said at least one transistor.
 21. An active terminationcircuit comprising at least one active termination branch and an activetermination output, said at least one active termination branchcomprising: at least one transistor for providing said activetermination output, a terminal of said at least one transistor beingcoupled to a current sink; an amplifier driving said at least onetransistor, said active termination output driving said amplifier tocontrol a current flowing through said at least one transistor toprovide said active termination output.
 22. The active terminationcircuit of claim 21, wherein said active termination output is providedat a drain of said at least one transistor.
 23. The active terminationcircuit of claim 21, wherein an inverting input of said amplifier iscoupled to a source of said at least one transistor.
 24. The activetermination circuit of claim 21, wherein said active termination outputdrives a non-inverting input of said amplifier through a feedbacknetwork.
 25. The active termination circuit of claim 24, wherein saidfeedback network comprises a feedback resistor coupled between saidactive termination output and said non-inverting input of saidamplifier.
 26. The active termination circuit of claim 24, wherein saidfeedback network comprises a feedback resistor coupled between groundand said non-inverting input of said amplifier.